Decontamination of radioactive metals

Abstract

Two alternate, mutually exclusive, methods of removing radio contaminants from metal are taught based respectively on electrowinning or electrorefining of the base metal. The alternative using electrorefining controls the anolyte oxidation potential to selectively reduce the technetium in the metallic feedstock solution from Tc(VII) to Tc(IV) forcing it to report to the anodic slimes preventing it from reporting to the cathodic metal product. This method eliminates the need for peripheral decontamination processes such as solvent extraction and/or ion exchange to remove the technetium prior to nickel electrorefining. The other alternative method combines solvent extraction with electrowinning. By oxidizing technetium to the heptavalent state and by using mixtures of tri-n-octyalphosphine oxide and di-2-ethyl phosphoric acid in aliphatic hydrocarbon carriers to extract the radio contaminants prior to electrowinning, the background metal may be recovered for beneficial reuse. Electrowinning may further polish the decontamination extraction process to remove residual actinides in solution while winning a radio- chemical free metal product. These methods are particularly useful for the decontamination of nickel by radio contaminants such as technetium and actinides.

What is claimed is: 
     
       1. A method of extracting technetium and actinide radiocontaminants from radiocontaminated nickel comprising the steps: (a) fabricating a nickel electrode contaminated with technetium and actinides; and then   (b) anodically dissolving the electrode contaminated with technetium and actinides in an oxidizing acid electrolyte solution to produce a solution containing actinide ions and at least 30 grams/liter of nickel and to oxidize the technetium to produce pertechnetate anions; and then   (c) removing pertechnetate anions and actinides by countercurrent solvent extraction with a barren solution containing TOPO, D 2  EHPA or mixtures thereof dissolved in an organic solvent, to produce a decontaminated, nickel containing raffinate, and a contaminated, loaded solvent stream; and then   (d) stripping the technetium values from the contaminated, loaded solvent stream with hydrochloric acid;   (e) passing the decontaminated, nickel containing raffinate through an absorbent for organic solvent; and then   (f) electrowinning the raffinate in an electrolysis cell with acidic electrolyte to remove residual actinides present, and to recover cathodic nickel.   
     
     
       2. The method of claim 1 using TOPO D 2  EHPA or mixtures thereof dissolved in an aliphatic hydrocarbon. 
     
     
       3. The method of claim 1 including providing a second extraction cycle utilizing further extraction of the primary extraction cycle raffinate to extract any cobalt isotopes which may be present. 
     
     
       4. The method of claim 1, where the oxidizing acid is selected from the group consisting of sulfuric acid and nitric acid, and where the pertechnetate anions and actinides removed in step (c) are stripped from the contaminated, loaded solvent stream by hydrochloric acid and incinerated. 
     
     
       5. The method of claim 1, where the barren solution extractant in step (c) contains (0.1 to 2)M TOPO (0 to 2)M D 2  EHPA dissolved in kerosine, and where the organic-to-aqueous phase contact ratios for the extraction are between 0.25 and 20. 
     
     
       6. The method of claim 1, where the absorbent used in step (d) removes residual organic before it passes into the electrolysis cell, and is a carbon column, and the electrolysis cell operates at a current density of 10 amp/ft 2  to 300 amp/ft 2  at a pH range of 1 to 6 for the electrolyte, with electrolyte additives selected from up to 30 g/L free sulfuric acid, up to 60 g/L boric acid and from about 20 g/L to 40 g/L chloride ions. 
     
     
       7. The method of claim 1, where spent acidic electrolyte from step (e) is recycled to step (b) for anodic dissolution. 
     
     
       8. A method of decontaminating radiocontaminated nickel, comprising the steps of: dissolving nickel contaminated with technetium and actinides in an oxidizing acid electrolyte solution to produce a solution containing at least 30 grams/liter of nickel ions contaminated with pertechnetate anions and actinide ions;   removing the pertechnetate anions and the actinide ions from the electrolyte solution by solvent extraction with an extractant dissolved in an organic solvent to produce a substantially decontaminated nickel-containing electrolyte solution and a contaminated organic solvent; and   electrowinning the nickel from the substantially decontaminated electrolyte solution.   
     
     
       9. The method of claim 8, wherein the oxidizing acid is selected from the group consisting of sulfuric acid and nitric acid. 
     
     
       10. The method of claim 9, wherein the pertechnetate anions and the actinide ions are extracted from the electrolyte solution with an extractant selected from the group consisting of TOPO, D2EHPA and mixtures thereof. 
     
     
       11. The method of claim 9, wherein the recycled electrolyte solution contains boric acid in amount of less than about 60 g/l. 
     
     
       12. The method of claim 9, wherein the recycled electrolyte solution contains from about 20 to about 40 g/l chloride ions. 
     
     
       13. The method of claim 9, wherein the recycled electrolyte solution contains from about 30 to about 50 g/l nickel. 
     
     
       14. The method of claim 13, wherein the recycled electrolyte solution contains a plating agent selected from the group consisting of boric acid, chloride ions and mixtures thereof. 
     
     
       15. A method of decontaminating radiocontaminated nickel, comprising the steps of: dissolving nickel contaminated with technetium and actinides in an oxidizing acid electrolyte solution to produce a solution containing at least 30 gm/liter or nickel ions contaminated with pertechnetate anions and actinide ions;   removing the pertechnetate anions and the actinide ions from the electrolyte solution; and   electrowinning the nickel from the decontaminated electrolyte solution.   
     
     
       16. The method of claim 15, wherein the oxidizing acid is selected from the group consisting of sulfuric acid and nitric acid. 
     
     
       17. The method of claim 15, wherein the solution is a sulfuric acid solution.